1. Field of the Invention
This disclosure concerns a method for the synthesis of PbSe nanowires in a non-coordinating solvent system.
2. Description of Related Art
One-dimensional nanowires possess many device fabrication advantages over their zero-dimensional nanocrystal counterparts. Primarily, these advantages stem from the potential to access the effects of quantum confinement in a structure with one-dimension that extends into the macroscopic regime. This combination of properties can ease difficulties associated with interfacing to a given material, yet still permit transport of electrons through a continuous inorganic structure without relying on hopping or tunneling to adjacent particles. Realization of this concept would be of substantial importance to many electronic applications. A particularly compelling material to examine for this purpose is PbSe, a narrow band gap semiconductor of interest for both photovoltaic and thermoelectric applications due to its large exciton Bohr radius and ability to absorb and emit infrared photons. This material has received significant attention as a potential candidate for exploiting multiexciton generation (MEG) in photovoltaic cells, and high aspect ratio structures may lead to improvements in these devices by reducing the rate of multiexciton Auger recombination.
Various methods have been reported for the formation of one-dimensional nanostructures, with the bulk of these relying on either electrochemical deposition in a pre-formed template or Vapor-Liquid-Solid (VLS) growth from a catalyst droplet on a solid support. The synthesis of nanowires in solution, analogous to the preparation of colloidal nanocrystals, has been less widely examined despite several potential advantages to the approach. This “bottom up” solution synthesis offers both the potential for smaller diameter structures, as well as scalability to obtain larger quantities of material, both important features for their eventual application. In addition, the organic ligands encapsulating these materials facilitate their manipulation in a variety of solvents, expanding possibilities for self-assembly and low cost device processing.
Several methods for the solution preparation of PbSe nanowires have been reported in the literature, but due to issues related to their reproducibility, dimensions, or purification, are not ideal for incorporation into a photovoltaic device. For example, the procedure reported by Lifshitz, et al. employs a KBH4 reduction that generates KCl and oxidized B salts, while Hull et al., utilize a Bi—Au catalyst particle to encourage one dimensional growth. These species represent additional components that must be separated from the product prior to utilization in a device as they could potentially interfere with electron transport. The procedure of Cho et. al. is more straightforward from an isolation standpoint in that the nanowire growth is reported to occur through an oriented attachment of PbSe nanocrystals, but in practice can be difficult to control due to the large volume of reagent that must be rapidly added to the reaction. Additionally, all three of these prior procedures are conducted at a relatively high dilution, increasing the volume of solvent required as the reaction is scaled up.